Field drydown of mature corn grain
Because grain drydown rates are greater when the drydown period is warmer, it stands to reason that a corn crop that matures in late August will dry down faster than one that matures in mid-September. In fact, there is a close relationship between the date when the grain nears physiological maturity (half-milkline or 2 to 3 weeks prior to kernel blacklayer) and the subsequent average daily drydown rate. Average daily drydown rates will range from about 0.8 percentage point per day for grain that nears maturity in late August to about 0.4 percentage point per day for grain that nears maturity in mid- to late September (Fig 3).
click image to zoomFig. 3. Relationship between field drying rate and the date at which the grain nears maturity (half-milkline) for three corn hybrids planted late April to early May, 1991-1994, westcentral Indiana. Bear in mind that grain moisture loss for any particular day may be quite high or low depending on the exact temperature, humidity, sunshine, or rain conditions that day. It is not unheard of for grain moisture to decline more than one percentage point per day for a period of days when conditions are warm, sunny, windy and dry. In contrast, there may be zero drydown of grain on cool, cloudy, rainy days.
Weather-Related Crop Stress and Field Drydown of Grain
Farmers often question whether field drydown will occur "normally" after some severe weather-related stress damages the crop prior to physiological maturity or causes premature death of the plants. Examples of such weather stress include damage caused by severe drought plus heat, late-season hail storms, and frost or killing freeze events prior to physiological maturity.
The answer in all cases to whether grain drydown will occur "normally" is essentially "yes", but this requires a bit of explanation.
Lingering severe stress such as drought or foliar disease (e.g., gray leaf spot) that occurs during the latter stages of the grain filling period typically causes premature death of the plants, smaller than normal kernels, AND premature formation of kernel black layer. The latter two factors usually result in earlier than expected drydown of the grain to the extent that grain moisture content in severely affected areas of a field is usually drier at harvest than lesser affected areas. The fact that grain drydown of the "prematurely mature" grain begins earlier usually means it occurs in relatively warmer time periods and so grain drydown rates per day are higher than would be expected if the grain had matured "normally" at a later date. However, the rate of grain drydown is "normal" for the time period during which the grain is drying.
NOTE: When areas of fields die prematurely due to stresses like drought, spatial variability for grain moisture at harvest can be dramatic and often creates challenges with the management of the grain dryer operation. This is especially true early in the harvest season when grain moistures of healthier areas of the field are in the low 20's. The spatial variability for grain moisture decreases later in the harvest season as grain moistures throughout the field settle to an equilibrium level (15% or less).